This disclosure relates to a combustor for a gas turbine engine and, more particularly, to a hole pattern formed in inner and outer combustor liners.
Gas turbine engines, such as those used to power modern commercial and military aircraft, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. The combustor generally includes radially spaced apart inner and outer liners that define an annular combustion chamber therebetween. Arrays of circumferentially distributed combustion air holes penetrate multiple axial locations along each liner to radially admit the pressurized air into the combustion chamber. A plurality of circumferentially distributed fuel injectors project into the forward end of the combustion chamber to supply the fuel for mixing with the pressurized air.
Combustion of the hydrocarbon fuel in the presence of pressurized air is know to produce a number of pollutants including nitrogen oxide (NOX) emissions that are subjected to stringent controls by regulatory authorities, and thus are sought to be minimized as much as possible.
At least one known strategy for minimizing NOX emissions is referred to as rich burn, quick quench, lean burn (RQL) combustion. The RQL strategy recognizes that the conditions for NOX formation are most favorable at elevated combustion flame temperatures, such as when a fuel-air ratio is at or near stoichiometric, for example. A combustor configured for RQL combustion includes three serially arranged combustion zones: a rich burn zone at the forward end of the combustor, a quench or dilution zone axially aft of the rich burn zone, and a lean burn zone axially aft of the quench zone.
During engine operation, a portion of the pressurized air discharged from the compressor enters the rich burn zone of the combustion chamber. Concurrently, the fuel injectors introduce a stoichiometrically excessive quantity of fuel into the rich burn zone. Although the resulting stoichiometrically fuel rich fuel-air mixture is ignited and burned to partially release the energy content of the fuel NOX formation may still occur.
The fuel rich combustion products then enter the quench zone where jets of pressurized air radially enter through combustion air holes from the compressor enter to the quench zone of the combustion chamber. The pressurized air mixes with the combustion products to support further combustion of the fuel with air by progressively deriching the fuel rich combustion products as they flow axially through the quench zone and mix with the air. Initially, the fuel-air ratio of the combustion products changes from fuel rich to stoichiometric, causing an attendant rise in the combustion flame temperature. Since the quantity of NOX produced in a given time interval is known to increase exponentially with flame temperature, substantial quantities of NOX may be produced during the initial quench process. As the quenching continues, the fuel-air ratio of the combustion products changes from stoichiometric to fuel lean, causing an attendant reduction in the flame temperature. However, until the mixture is diluted to a fuel-air ratio substantially lower than stoichiometric, the flame temperature remains high enough to generate considerable quantities of NOX.
Finally, the deriched combustion products from the quench zone flow axially into the lean burn zone. Additional pressurized air in this zone supports ongoing combustion to release energy from the fuel. The additional pressurized air in this zone also regulates the peak temperature and spatial temperature profile of the combustion products in attempts to reduce turbine exposure to excessive temperatures and excessive temperature gradients.
One known example pattern of combustion air holes in the inner and outer liners includes a series of holes that have two different sizes, i.e. a major hole and a minor hole. The pattern includes a major hole diameter and a minor hole diameter that alternate with each other. A minor hole on one of the inner and outer liners is radially aligned in a major hole of the other of the inner outer liners. This alternating pattern continues about a circumference of each of the inner and outer liners. While this pattern has been effective in reducing NOX emissions, still greater improvement is desired as environmental regulations become increasingly more strict with regard to such emissions.